In many chemical processes it is desirable to condense vapor flow from a distillation column and reflux the condensate to the column in order to accomplish the desired separation. One such chemical process is for the manufacture of oxoalcohols. In some cases this is accomplished by installing a condenser on top of the distillation column. The condensate from the condenser then can flow by gravity back to the top trays of the column. The advantage of this arrangement is reduced capital cost and operating cost because of the elimination of the need for reflux pumps, and consequently eliminating the power required to operate the pump(s) over the life of the system.
Without a pump the condensate flow cannot be effectively measured because there is insufficient pressure drop available in the gravity flow line from the liquid accumulator section of the condenser back to the column to meet the minimum requirements of a differential pressure cell type measurement device. Accordingly up until now, when choosing to mount a condenser on top of the column it was then predetermined to operate the column without effective and useful measurement of the condensed liquid flow (reflux) from the condenser. The column operation, therefore, cannot be fully optimized for energy usage and product quality.
Prior to the invention operators had to manage the column in a manual mode both during steady state operation and during upsets. The steam usage was higher than needed to achieve the desired separation since there was no way to optimize it. This excessive steam overloaded the condenser when the column feed rate stopped or significantly decreased.
The prior art discloses methods and apparatus for controlling fractional distillation apparatus. Scovill U.S. Pat. No. 2,398,807, issued Apr. 23, 1946, is typical of the prior art. In Scovill, there is shown a fractionating column having a distilling chamber and a condensing chamber above the distilling chamber. A collector is arranged to catch liquid from the condenser and direct it into a calibrated chamber. The chamber is provided with a valve for opening and closing flow from the chamber. When it is desired to measure rate of flow of the reflux the chamber is closed and reflux is collected. The filling of the chamber may be timed by viewing through a sight glass and the rate of flow thus ascertained. This results in unsteady operation of the rectifying section of the distillation column.
In Fickel U.S. Pat. No. 3,881,994, issued May 6, 1975, there is described a control system for regulating heat input to the reboiler section of a distillation column. Flow-measuring means for determining the quantity of vapor passing into the fractionation section is internally disposed within the reboiler section and provides a signal which is transmitted to the reboiler heater thereby regulating the degree of vaporization to be effected therein.
Howard U.S. Pat. No. 1,049,718, issued Jan. 7, 1913, shows a standpipe with lateral discharge orifices which serve as a means for indicating flow of liquid therethrough by noting the number of discharge orifices through which liquid flows.
Plaster U.S. Pat. No. 3,053,521, issued Sep. 11, 1962 shows a method to control the product removal from a side draw tray by using a weir assembly within the distillation zone of the column.
Nothwithstanding the teaching of the above prior art, there exists a long felt need for a method and apparatus for measuring the amount of condensate in a gravity flow reflux to optimize operation of a distillation column, by regulating the heat input to the column to give only the desired degree of separation.